If one more person mentions long term engine damage i will track you down and bludgeon you with my wireless mouse..
its like talking about long term damage to shoe leather or the tread on your car tyres.
blades are replaced when they meet the time and cyc limits defined as an LLP (Life Limited Part) they are also inspected at intervals as well as trend monitoring, if there is any question as to the longevity they are replaced prior to LLP limits, Also with AD's and SB's they are often overhauled or replaced if they don't meet limits prior to intervals anyway.
No, it's not like that at all. Just like with shoes, you expect engines to wear out from normal use, but no-one knows how accelerated any wear may be, if there is any increase at all, with varying levels of ash contamination. To use your analogy, it's like expecting a normal pair of shoes to stand up to walking across a rough lava field - they just won't last as long, but you can't predict how quickly they'll wear out.
What is certain is that the ash will build deposits that do more than just cause wear - they will cause hot spots and thermal stresses or melting of components as well as affect the internal thermodynamics of the engines. How rapidly these deposits form is unknown. What level of deposit is safe is unknown. How big these deposits can grow before detaching is unknown. the level of damage caused by deposits breaking off is unknown... Therein lies the problem; there are too many unknowns to determine if flying through this amount of predicted contamination is safe.